1. Field of the Invention
This invention relates to a gasket composition which is especially capable of sealing at low flange pressures.
Numerous materials are known which are suitable for use in gaskets to provide a seal between contiguous or abutting members or parts. Such gaskets are employed to seal against fluid (air and liquid) leaks in applications such as cylinder heads and valves. In these applications, it is necessary and desirable for the gasket to be capable of sealing against fluid leaks at low operational flange pressures.
Characteristics desirable for a serviceable gasket in addition to giving a good seal at flange pressures as low as 500 psi, include: good compressive strength up to 2,000 psi and good compression/recovery. A gasket should therefore be durable for the stresses experienced during service, resistant to the temperatures which it experiences and it should be flexible but resistant to mechanical deformation.
According to the instant invention there is provided a water laid gasket composition which has good compression/recovery, and is capable of giving a superior seal against fluid leaks at low flange pressures.
2. Brief Description
The instant gasket composition is a substantially homogeneous mixture of intermeshed fibers, expanded and/or expandable polymeric microspheres and a binder, wherein the microspheres expanded inside the gasket sheet after it was formed. In preferred embodiments, a filler is also included.
The polymeric microspheres are incorporated with the other ingredients as unexpanded, inflatable microspheres. They are then inflated to a larger volume at some time after the gasket material is formed. The gasket material is "formed" when it is wet laid as a sheet. The microspheres can then be inflated by exposing them to the minimum amount of heat needed for expansion.
Polymeric microspheres which can permissively be used includes: A) Thermoplastic microspheres which contain a volatile blowibng agent such as a solid or liquid which becomes a gas at a certain termperature; when an effective amount of heat is used the blowing agent causes the outer layer to inflate; frequently, the outer layer must be softened with heat or steam for optimum expansion; and B) Microspheres of polymeric foam which also expand when heated, typically due to the action of a blowing agent, can also be used.
Procedures for the preparation of microspheres which expand upon exposure to specific levels of heat are known in the art and such spheres are also commercially available.
Any other ingredients desired or needed for the instant gaskets, such as fiber, filler, and binder can be selected from anyy of these materials which are available. Preferred combinations can be formed using fibers, fillers, and binders which are typically used in the gasketing industry to achieve specific characteristics; or for specific applications.
The instant gasket is formed by mixing the microspheres with the fiber in an aqueous slurry with agitation. After preparing a suspension, the beater addition water-laid gasket preparation can then be used. In the preparation forming the wet gasket sheet, the fibers, microspheres, binder, and, if desired, a filler and additives such as antioxidants are flocculated out of an aqueous suspension using a flocculant and a base and then water removal (dewatering). Water removal usually includes draining and wet-laying into a sheet and roll pressing the wet sheet squeezing out more water. This sheet forming procedure advantageously insures the uniform distribution of the microspheres in the gasket sheet.
The unexpanded, inflatable microspheres can be expanded during or after gasket drying by using the amount of heat needed to cause microspheric expansion. Usually a minimum of about a 100.degree. C. temperature will be required for expansion. Advantageously, the microspheres expand to fill the cavities or voids which tend to develop in drying gasket material. Since the microspheres, moreover, are substantially uniform in distribution throughout the gasket, the volumetric increase of the microspheres allows this pore-filling to be similarly uniform throughout the gasket.
The term "internal densification" is used herein to refer to the decrease in or elimination of open space (pores or cavities) within the gasket by the expansion of the microspheres inside the gasket material afterits formation. The expanding microspheres take up at least a portion of the internal cavity areas which are typically present as hollow spaces within the gasket. The microspheres therefore make the gasket less porous. In operation, these cavities or spacees are less able to allow passage of fluid (either liquid or gas). Thus, the internal densification will operate to provide both a more effective seal, and a good seal at lower pressures. Since the microspheres expand into the internal cavity areas of the gasket, effective service and a good seal are still obtainable even if operating pressures should cause microspheric rupturing.
The term "external densification" refers to the application of pressure to the outside of the gasket by pressing or calendering it in a uniform manner to press the gasket material into a thinner and more compact sheet. This decreases the gasket volume, making the gasket more dense.
In a preferred embodiment of the instant invention, the inflation of the microspheres occurs during the drying step by using a drying temperature which will cause expansion of the spheres. Since many of the internal cavities develop during the drying step, this embodiment allows the internal densification to occur as cavities develop.
Other embodiments provide for the expansion of the microsphere: after drying; before, during or after the gasket is subjected to external densification; and/or during use. High temperature expanding spheres are used in a gasket material when it is desired that the microspheric expansion occur when gasket is in use.
It is permissable to include more than one type of microsphere. Different types of unexpanded microspheres can be included which expand at different temperatures, and/or in different amounts can be used. By using two or more different types of microspheres which expand at different temperature levels, expansion can occur at more than one time. Thus, for example, expansion of microspheres could occur both during drying and as the gasket is being used by using a high-temperature expansion sphere. The uniform distribution inside the gasket insues that the volume change of the spheres occurs throughout the gasket in a substantially uniform manner even if different types of microspheres are used which expand in different amounts and/or at different times.